Studies will be conducted to define the molecular properties of the regulatory proteins in the thin filaments of the heart and to relate these properties to cardiac contractile control. Attempts will be made to crystallize the troponin subunits or the whole troponin complex. The amino acid side chains of troponin and tropomyosin will be chemically modified in order to define the regions of the polypeptide chains which are involved in subunit interaction and biological activity. BIBLIOGRAPHIC REFERENCES: Strasburg, G.M. and M.L. Greaser. 1976. The native subunit pattern of tropomyosin. FEBS Letters 72:11. Greaser, M.L., M. Yamaguchi, and G. Vanderkooi. 1977. Crystal forms of alpha-s-tropomyosin. J. Mol. Biol., in press.